The Leviathan is mentioned five times in the OT, in
Job 3:8,
Job 40:15–41:26,
Psalm 74:14,
Psalm 104:26 and
Isaiah 27:1.
The leviathan is one of three legendary monsters from ancient Jewish folklore, the other two are behemoth and ziz. Many might not be immediately famliar with the ziz, its only mention in the Bible is found a handful of times, though it is often translated into English as "wild beasts", as we read in Psalm 50:11
"
I know all the birds of the mountains, and ziz siday is Mine." - Psalm 50:11
English translators seem to have generally speaking favored rendering ziz based on an ambiguous root possibly meaning "to move" or "moving thing(s)", and thus have chosen "wild beasts". Siday is translated as "of the field", from sadeh, "field" or "plain".
Given the context though, seeing as God says He owns the birds of the mountains, and also the ziz of the field, that a reference to a bird here might be in keeping with the intended meaning, rather than "wild beasts". Though it's possible a case could be argued either way.
In any event, these are the three legendary creatures that are mentioned in the Bible (depending on how one wants to treat the case of the ziz). And they likely do not necessarily correspond to any real living creatures, but are rather references to fantastical, legendary animals. The ziz, for example, in traditional Jewish/Hebrew folklore is very similar to both the Persian simurgh or the Greek phoenix.
That said, it may be that, minus the fantastical elements ascribed to these creatures, they may have originally been based on real animals. For example the behemoth has been described as perhaps having been an elephant, or at least based on the elephant, with its "tail" referring to the animal's trunk, or as some scholars think,
something else altogether different, cough cough, hint hint, if you catch my drift. And likewise, some have very often thought that leviathan was probably, or at least based on, the crocodile.
That said, there's really no reason to think that modern cryptids have any relationship to the Hebrew fantastical beasts.
I'd love nothing more if the loch ness monster, or some other really amazing cryptid were real--but they are almost certainly not real, without any more evidentiary support behind them than UFOs having little green men visiting earth, or any other modern tall tales.
-CryptoLutheran